1H-Benzo[g]pteridine-2,4-dione

The structure of the title compound, reported from powder diffraction data and 15N NMR spectroscopy, is confirmed using low-temperature data from a twinned crystal. The tautomer in the solid state is alloxazine (1H-benzo[g]pteridine-2,4-dione) rather than isoalloxazine (10H-benzo[g]pteridine-2,4-dione).


Structure description
1H-Benzo[g]pteridine-2,4-dione, popularly known as alloxazine, is a tautomer of isoalloxazine (10H-benzo[g]pteridine-2,4-dione), the same ring system that is present in riboflavin, flavin nucleotides (FMN and FAD), and flavoproteins. Unlike nicotinamide coenzymes, NAD(P) + and NAD(P)H, flavin nucleotides serve in both oneelectron and two-electron transfer reactions because the isoalloxazine ring can exist in several different ionization and/or redox states (Massey & Hemmerich, 1980). Further, the strong but mostly non-covalent interactions within the flavoprotein binding site allow the fine-tuning of the redox chemistry of the isoalloxazine ring system (Ghisla et al., 1974;Hu et al., 2015;van den Heuvel et al., 2002) which, among many things, helps in minimizing the 1-electron reduction of molecular oxygen to the superoxide anion radical. It is believed that the spatial arrangement of the reacting oxygen molecule may have a direct bearing on the outcome of a flavoprotein serving as an oxidase or dehydrogenase function (Chaiyen et al., 2012), a process that can be mimicked in simple chemical model data reports systems of phenazine reacting with NAD(P)H in micelle forming surfactant solutions (Nishikimi et al., 1972;Rao, 1989a,b;Uppu, 1995). While there have been several efforts to define flavin-protein interactions that have mainly capitalized on differences in the chemical reactivity of the protein-bound flavin, we were surprised to note that, except for one recent study by Smalley et al. (2022), there are hardly any studies of the crystal structure of alloxazine itself.
In view of the above and since two-thirds of flavoprotein allelic variants are linked to human diseases (Lienhart et al. 2013), we determined the crystal structure of alloxazine using a Bruker Kappa APEXII DUO diffractometer. Using lowtemperature (90 K) data from twinned crystals, our results confirm the observations of Smalley et al. (2022), who used powder diffraction data along with 15 N NMR spectroscopy. The tautomer in the solid state is alloxazine rather than isoalloxazine. The N-bound hydrogen atoms were located and their positions were refined in order to confirm the tautomer. The molecule, shown in Fig. 1 is nearly planar, with an r.m.s deviation for 16 non-hydrogen atoms of 0.015 Å and a maximum deviation of 0.025 (3) Å for C5.

Figure 2
The hydrogen-bonded chain.

Figure 3
The unit cell, showing the offsets of hydrogen-bonded dimers.

Synthesis and crystallization
The title compound, C 10 H 6 N 4 O 2 (alloxazine) was obtained from Sigma-Aldrich, St. Louis, Missouri, USA and was used without further purification. Single crystals in the form of pale yellow plates were prepared by slow cooling of a nearly saturated solution of alloxazine in dimethyl formamide at 135 AE 2 C

Refinement
Crystal data, data collection and structure refinement details are summarized in  where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max < 0.001 Δρ max = 0.26 e Å −3 Δρ min = −0.28 e Å −3 Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.

data-2
IUCrData (2023). 8, x221223 Refinement. Refined as a 2-component twin, by 180 deg. rotation about reciprocal 0 0 1. Refinement was vs. an HKLF 5 file prepared using TWINABS. The refined BASF parameter is 0.446 (4). Seven outlier reflections were omitted from the refinement. All H atoms were located in difference maps and those on C were thereafter treated as riding in geometrically idealized positions with C-H distances 0.95 Å. Coordinates of the N-H hydrogen atom were refined. U iso (H) values were assigned as 1.2U eq of the attached atom.
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )